This invention relates to a power supply apparatus used for an electrosurgical device such as an electric scalpel.
With an electric scalpel, a local high-frequency current is caused to flow through the tissue of a living body, such as a human body, and incision and stanching are conducted by utilizing Joule heat given to the tissue by the local current. Such an electric scalpel is provided with a plate electrode forming one pole of a high-frequency current path and a therapeutic electrode forming the other pole of the current path to supply the local current. The therapeutic electrode is in the form of a needle or sword having quite a narrow contact area, and the local current, i.e. a surgical current, is concentrated in close vicinity to the tissue of a living body touched by the therapeutic electrode. A small explosion of gas is caused within the tissue by the Joule heat produced during such concentration of the current, and a portion of the tissue surrounding the therapeutic electrode is incised or resected. Since protein on the incised or resected surface can be thermally coagulated, lymphatic vessels and capillaries can be blocked up for hemostasis.
Usually, a coagulation current or surgical current for the aforesaid hemostasis or stanching is obtained by switching on and off the power circuit for a power amplifier in the power supply apparatus. A semiconductor switch circuit or the like is used for switching on and off the power circuit. However, if the switch circuit breaks down to be rendered ever-on, then a continuous great current will be caused to flow through the therapeutic electrode of the electric scalpel. If such breakdown of the switch circuit occurs in stanching, a great enough amount of energy to incise the tissue will be discharged to the vicinity of the therapeutic electrode, possibly resulting in unexpected incision. Such disagreeable situation can be avoided if an element forming the switch circuit, e.g. a transistor switch, is absolutely unbreakable. When intermittently conducting great current, however, semiconductor elements are liable to deteriorate, and the most reliable ones may suddenly be broken during use.